Method and circuit for storing and providing historical physiological data

ABSTRACT

Embodiments of the present invention include systems and methods that relate to pulse oximetry. Specifically, one embodiment includes an oximeter sensor comprising a light emitting element configured to emit light, a light detector configured to detect the light, a resistor having a resistance value that is detectable by a monitor, and a memory storing a contact resistance value of a connector that is disposed between the monitor and the resistor when the monitor and the oximeter sensor are communicatively coupled.

This application is a continuation of U.S. patent application Ser. No.10/452,806, filed May 30, 2003, which is a divisional application ofU.S. patent application Ser. No. 09/943,899, filed Aug. 30, 2001, whichissued as U.S. Pat. No. 6,606,510 and claims the benefit of U.S.Provisional Application Ser. No. 60/299,616, filed Aug. 31, 2000, all ofwhich are incorporated herein by reference.

BACKGROUND

Pulse oximetry is typically used to measure various blood flowcharacteristics including, but not limited to, the blood oxygensaturation of hemoglobin in arterial blood, the volume of individualblood pulsations supplying the tissue, and the rate of blood pulsationscorresponding to each heartbeat of a patient. Measurement of thesecharacteristics has been accomplished by use of a non-invasive sensorthat passes light through a portion of a patient's blood perfused tissueand photo-electrically senses the absorption and scattering of light insuch tissue. The amount of light absorbed is then used to estimate theamount of blood constituent in the tissue. The “pulse” in pulse oximetrycomes from the time varying amount of arterial blood in the tissueduring the cardiac cycle. The signal processed from the sensed opticalsignal is a familiar plethysmographic waveform due to cycling lightattenuation.

To estimate blood oxygen saturation of a patient, conventionaltwo-wavelength pulse oximeters emit light from two light emitting diodes(LEDs) into a pulsatile tissue bed and collect the transmitted lightwith a photodiode (or photo-detector) positioned on an opposite surface(i.e., for transmission pulse oximetry) or an adjacent surface (i.e.,for reflectance pulse oximetry). One of the two LEDs' primary wavelengthis selected at a point in the electromagnetic spectrum where theabsorption of oxyhemoglobin (HbO₂) differs from the absorption ofreduced hemoglobin (Hb). The second of the two LEDs' wavelength isselected at a different point in the spectrum where the absorption of Hband HbO₂ differs from those at the first wavelength. Commercial pulseoximeters typically utilize one wavelength in the near red part of thevisible spectrum near 660 nanometers (nm) and one in the near infrared(IR) part of the spectrum in the range of 880-940 nm. The amount oftransmitted light passed through the tissue will vary in accordance withthe changing amount of blood constituent in the tissue and the relatedlight absorption.

An encoding mechanism is shown in U.S. Pat. No. 4,700,708, thedisclosure of which is incorporated herein by reference. This mechanismrelates to an optical oximeter probe which uses a pair of light emittingdiodes (LEDs) to direct light through blood perfused tissue, with adetector picking up light which has not been absorbed by the tissue. Theoperation depends upon knowing the wavelength of the LEDs. Since thewavelength of LEDs can vary, a coding resistor is placed in the probewith the value of the resistor corresponding to the actual wavelength ofat least one of the LEDs. When the oximeter instrument is turned on, itfirst applies a current to the coding resistor and measures the voltageto determine the value of the resistor and thus the value of thewavelength of the LED in the probe.

Oxygen saturation can be estimated using various techniques. In onecommon technique, the photo-current generated by the photo-detector isconditioned and processed to determine the modulation ratio of the redto infrared signals. This modulation ratio has been observed tocorrelate well to arterial oxygen saturation. The pulse oximeters andsensors are empirically calibrated by measuring the modulation ratioover a range of in vivo measured arterial oxygen saturations (SaO₂) on aset of patients, healthy volunteers, or animals. The observedcorrelation is used in an inverse manner to estimate blood oxygensaturation (SpO₂) based on the measured value of modulation ratios of apatient. The estimation of oxygen saturation using modulation ratios isdescribed in U.S. Pat. No. 5,853,364, entitled “METHOD AND APPARATUS FORESTIMATING PHYSIOLOGICAL PARAMETERS USING MODEL-BASED ADAPTIVEFILTERING”, issued Dec. 29, 1998, and U.S. Pat. No. 4,911,167, entitled“METHOD AND APPARATUS FOR DETECTING OPTICAL PULSES”, issued Mar. 27,1990. The relationship between oxygen saturation and modulation ratio isfurther described in U.S. Pat. No. 5,645,059, entitled “MEDICAL SENSORWITH MODULATED ENCODING SCHEME,” issued Jul. 8, 1997. All three patentsare assigned to the assignee of the present invention and incorporatedherein by reference.

Nellcor U.S. Pat. No. 5,645,059, the disclosure of which is herebyincorporated herein by reference, teaches coding information in sensormemory used to provide pulse modulated signal, to indicate the type ofsensor (finger, nose), the wavelength of a second LED, the number ofLEDs, the numerical correction terms to the standard curves, and anidentifier of the manufacturer.

The LEDs and photo-detector are typically housed in a reusable ordisposable oximeter sensor that couples to the pulse oximeterelectronics and the display unit (hereinafter referred to as themonitor). The sensors are often connected to patients for long periodsof time. Conventionally, historical physiological data for the patientis collected, if at all, by the monitor coupled to the sensor. Thehistorical data can be valuable to a clinician or medical personnel fordiagnostic and monitoring purposes. Patients are often moved to variouslocations during treatment. For example, a patient may be picked up inan ambulance, delivered to an emergency room, moved to an operatingroom, transferred to a surgical recovery room, transferred to anintensive care unit, and then moved to a nursing floor or otherlocations. Thus, the patient may be moved between various locationswithin the same hospital, or between different hospitals. In manyinstances, the sensor employed to monitor the condition of the patientis adhesive in its attachment and remains with the patient. Themonitors, however, are typically local to particular locations within afacility or vehicle. The sensor is normally disconnected from themonitor at a departure site and reconnected to another monitor at adestination site. Consequently, any patient related data (e.g.,historical physiological data) collected by the monitor at the departuresite is normally unavailable to the clinician attending the patient atthe destination site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an exemplary embodiment of an oxygenmeasurement system; and

FIG. 2 shows a block diagram of an exemplary embodiment of an oxygenmonitor and sensor; and

FIG. 3 shows a block diagram of a pulse oximeter system incorporating anexemplary calibration memory element according to an embodiment.

DETAILED DESCRIPTION

The present techniques relate to physiological monitoring instrumentsand, in particular, sensors that include a mechanism for storing andproviding to a monitor historical physiological data such as bloodoxygen saturation data. Embodiments of the present invention includepulse oximetry sensors that include coded information relating topatients.

The present techniques provide a memory chip for use in an oximetersensor, or an associated adapter or connector circuit to provideenhanced capabilities for the oximeter sensor. The techniques provide amechanism for storing and providing patient related data such as patienttrending data, a patient ID or historical physiological data (e.g.,blood oxygen saturation data for a patient).

In addition to providing unique data to store in such a memory, thepresent techniques include unique uses of the data stored in such amemory. In a specific embodiment, the patient related data (e.g.,historical physiological data) is stored in a storage medium that“travels” with the patient and is accessible wherever the patient ismoved. This is achieved by storing the data within a sensor assembly,(e.g., within the sensor itself, a connector plug, a connector cable, oran interconnection module). At the destination site, a monitor or adevice capable of interfacing with the sensor assembly electronics canretrieve and display the data. The stored data allows a clinician ormedical personnel at the destination site to assess the condition of thepatient for the entire time that the patient has been previouslymonitored. The techniques can be used to store and provide various typesof physiological data including, but not limited to, patient trendingdata, a patient ID, blood oxygen saturation, heart rate, blood pressure,and temperature data.

A specific embodiment provides a pulse oximeter sensor that includes anumber of light sources, at least one photo-detector, and a memorycircuit associated with the sensor. The light sources are selected tooperate at different wavelengths. The photo-detector receives lightemitted by the plurality of light sources. The memory circuit storespatient related data. For example, the memory circuit storesphysiological data derived from the detected light and sent to thecircuit by an oximeter monitor, and the circuit then provides the datalater when requested by a monitor. The physiological data is indicativeof a physiological condition of a patient being monitored by the sensor.

Another specific embodiment provides a method for storing patientrelated data (e.g., physiological data). The method detects, via asensor, at least one signal indicative of a physiological condition, andconditions the detected signal to generate data samples. The datasamples are processed to generate the physiological data, wherein thephysiological data describes the physiological condition. Thephysiological data is stored within a memory associated with the sensor.The physiological data can be coded and compressed before storage to thememory. Other types of patient data (e.g., patient ID) are stored on thememory in other embodiments.

Part I

FIG. 1 shows a perspective view of an exemplary embodiment of aphysiological measurement system 100. System 100 includes a monitor 110.Monitor 110 couples via an electrical cable 128 to a sensor 130 that isapplied to a patient 132. Sensor 130 includes a sensor cable 124 and asensor connecting plug 122. Sensor 130 may include light sources (e.g.,LEDs) and a photo-detector along with suitable components to couple theelectro-optical components to electrical cable 128. Sensor 130 is shownin FIG. 1 as a clip-on sensor. However, present embodiments can beapplied to many sensor implementations, including those attached to apatient by adhesive and other attachment means. In a specificembodiment, monitor 110 is a pulse oximeter and sensor 130 is a pulseoximeter sensor. However, the sensor may include a temperature, heartrate, blood pressure, or other type of physiological sensor.

In an exemplary embodiment, for estimating blood oxygen saturation,light from light sources at two or more wavelengths (e.g., red andinfrared) is transmitted through a patient's blood perfused tissues(e.g., in a finger) and detected by a photo-detector. The selection ofthe wavelengths is based on a number of factors. Such factors includethe absorption characteristics of the patient and transmission medium.The light sources and photo-detector are typically housed within asensor that couples to the monitor (e.g., the pulse oximeter). Thedetected optical signal is then provided to the monitor for processing.

FIG. 2 shows a block diagram of an embodiment of monitor 110 and sensor130. Within monitor 110, a time processing unit (TPU) 220 providescontrol signals 222 to an LED driver 224 that, via data line(s) 226,alternately activates LEDs 230 within sensor 130. Depending on theparticular implementation, LEDs 230 include two or more LEDs and LEDdriver 224 provides the drive signals for the LEDs. When activated, thelight from LEDs 230 passes through a medium (e.g., air or a fiber opticcable, depending on the implementation) into a patient's tissues 234.After being transmitted through or reflected from the tissues, the lightis received by a photo-detector 240 via another medium (e.g., air oranother fiber optic cable). Photo-detector 240 converts the receivedlight into a photo-current, which is then provided to an amplifier 250that amplifies the photo-current.

As shown in FIG. 2, the amplified signal from amplifier 250 is providedto circuitry for two different channels, one channel for each of the redand infrared wavelengths. For a three-wavelength implementation,circuitry is provided for three channels. Each channel circuitryincludes an analog switch 252 coupled in series with a low pass filter254 that is further coupled in series with an analog-to-digitalconverter (ADC) 256. Control lines 258 from time processing unit 220select the sampled data from the channel corresponding to the LED beingactivated. Specifically, the sampled data from ADC 256 a is selectedwhen the red LED is activated and the sampled data from ADC 256 b isselected when the infrared LED is activated. The sampled data from ADCs256 is provided to a buffer 260 that stores the data for furtherprocessing. In an implementation, as buffer 260 periodically fills up, aprocessor 262 coupled to a bus 264 directs the transfer of the data frombuffer 260 into a memory 266. The monitor implementation shown in FIG. 2is one of many implementations. The present techniques can be adaptedfor application in various monitor implementations.

The sensor 130 may include circuitry that stores historicalphysiological data and provides the data when requested. As shown inFIG. 2, sensor 130 includes a memory 236 coupled to an interface circuit238. Interface circuit 238 provides signal conditioning, and can alsoprovide other functions such as address decoding, and so on. Interfacecircuit 238 couples via a bus 270 to a data interface circuit 268 withinmonitor 110. Through interface circuits 238 and 268, physiological datais transferred between monitor 110 and sensor 130.

In an embodiment, to enhance compatibility of the present sensor withconventional sensors and conventional monitors, bus 270 is implementedusing new signal lines (i.e., not using or sharing the existing signallines of conventional sensors). Bus 270 can be implemented as a serialbus, a parallel bus, or other bus architectures. With thisimplementation, when sensor 130 is plugged into a monitor not capable ofsupporting its features, the signals on interface circuit 238 are simplyignored by the monitor, or alternatively not requested by the monitor.

In another embodiment, interface circuits 238 and 268 interact viasignal line(s) or wire(s) existing in conventional sensors and monitors.For example, interface circuits 238 and 268 can couple via data line(s)226 and time multiplex with the LED drive signals from LED driver 224.

Time processing unit 220, buffer 260, processor 262, memory 266, anddata interface circuit 268 can be implemented in various manners. Forexample, these elements can be implemented within a single integratedcircuit, such as a DMC68HC 16 micro-controller from Motorola. Theseelements can also be implemented within an application specificintegrated circuit (ASIC), a digital signal processor, amicrocontroller, or other circuits.

In one embodiment, patient-specific data such as trending data orpatient monitoring parameters can be actively stored in the memory 236(e.g., a memory chip). As the patient and sensor travel fromward-to-ward of the hospital, and consequently plug into differentoximeters, the patient-specific data can be read from memory 236 of thepatient's dedicated sensor and displayed on a display screen for viewingor used by the oximeter monitor for other purposes. Memory 236 may, forexample, be implemented as a random access memory (RAM), a FLASH memory,a programmable read only memory (PROM), an electrically erasable PROM, asimilar programmable and/or erasable memory, any kind of erasablememory, a write once memory, or other memory technologies capable ofwrite operations. Examples of patient specific data that can be storedin memory 236 are now discussed.

Patient trending data regarding the history of a patient's blood oxygensaturation (SpO₂) level, pulse rate, pulse amplitude, perfusion data,and other patient data over a period of time can be recorded in memory236. The oximeter monitor can continuously or periodically store apatient's current trend data into memory 236 to maintain a historicaldata for the patient. The memory can be erased and overwritten multipletimes. For example, the patient trend data can be erased from memory 236each time a sensor is used on a new patient (e.g., each time theoximeter monitor is turned off or when user input to the monitorindicates a new patient). This memory characteristic may beadvantageous, for example, for nondisposable sensors that may be usedmultiple times on multiple patients. Specific examples of memory devicesthat can be erased and overwritten are Flash, EEPROM, battery backedRAM, and other technologies. Alternatively, the data encoded into memory236 can be permanent and non-erasable. In a specific embodiment, topreserve the historical data and prevent accidental erasure, the sensormemory can be written only once. This memory characteristic alsoprevents erasure of the data during sensor operation. Further details ofa Method and Circuit for Storing and Providing Historical PhysiologicalData are discussed in U.S. patent application Ser. No. 09/520,104 toSwedlow et al., filed Mar. 7, 2000, which is incorporated by referenceherein in its entirety.

As another example, the lowest and/or highest blood oxygen saturationlevel, pulse rate, pulse amplitude value, temperature data, bloodpressure, perfusion data, or any other patient data during the monitoredtime may be stored in memory 236 by the oximeter monitor. If desired,the lowest/highest values of these patient parameters over a pastspecified monitoring time (e.g., 2 hours, 1 day, etc.) may be recordedin memory 236.

Expected ranges for patient parameters (such as pulse rate, pulseamplitude, and blood oxygen saturation level) that are specific to aparticular patient may also be recorded in memory 236 by a clinician.This can be a desirable feature, because the expected patient trendingdata can vary significantly for each patient. The oximeter monitor cancompare the expected range for the patient stored in memory 236 with themonitored patient trending data to determine if the patient's pulse andblood oxygen levels are within the expected range for that patient. Ifthe monitored patient parameter varies outside the patient-specificrange recorded in memory 236, a warning message may be displayed on theoximeter monitor or an alarm signal may be sounded. If desired, anyvariations in the monitored patient parameters from the expected rangesmay be recorded in memory 236 along with a time stamp.

If desired, portions of a patient's medical chart and/or past medicalhistory can be digitally encoded and stored in memory 236 (if sufficientmemory space is available) so that this information is maintained withthe patient as he is moved around and can be easily accessed anddisplayed using an oximeter monitor if the patient transferred to adifferent room or hospital. During normal operation, when the sensor isplugged into the monitor, the monitor receives the signal from thephoto-detector within the sensor and processes this signal to obtain thedesired physiological data. In some prior art conventional monitors, thephysiological data is stored in a memory within the monitor andretrieved at a later time by a caregiver when requested. However, when apatient is moved to new locations and different monitors are used, thedata stored in the monitor at the previous site is typically notavailable at the current site, and the historical data is unavailable.

In accordance with the present embodiments, the physiological data isprocessed, displayed, and stored in the monitor in the normal or usualmanner. In addition, the data is compressed and provided to the sensorfor storage in the memory 236 associated with the sensor. Alternatively,uncompressed data can be provided to and stored in the memory 236. Whenthe sensor is later plugged into another monitor, the new monitor canretrieve the data stored in the sensor memory, decompress the retrieveddata, and display the decompressed data. In an embodiment, when thesensor is first plugged into a new monitor, the monitor retrieves anddisplays the historical physiological data for the most recentpredetermined period (i.e., the last 20 or 30 minutes). Thispredetermined period can be programmed by the clinician or can bepreprogrammed into the sensor memory.

Alternatively, the monitor can be configured to retrieve and display thehistorical physiological data at any time upon request by a health caregiver (or a clinician), by the health care giver simply activating acontrol knob on the monitor. The monitor can be preset so as toautomatically retrieve the data upon occurrence of a predeterminedevent, such as a sensor being plugged into the monitor, or can bepreconfigured so that the data is only retrieved upon explicit commandby a health care giver.

The pulse oximeter can keep track of how long a particular patient hasbeen monitored by the pulse oximeter and can periodically store thattime interval in memory 236 by checking the elapsed time on a counter.For example, a time stamp of the data can be stored. In this case, thefirst data sample includes the specific time (e.g., date and time) whenthe data is recorded. Subsequent data samples can be indicated by thenumber of epochs away from the first (or a previous) data sample. Thecounter may be a circuit element in the oximeter monitor that is reseteach time the oximeter monitor begins to receive data signals from asensor or each time that the oximeter monitor is turned off. The timeperiod that a patient has been monitored by the oximeter sensor may bedisplayed on a display screen for viewing.

The pulse oximeter monitor may also include a digital clock that keepstrack of the current date and time. The date and time that the oximetermonitor was turned on and the date and time that the oximeter monitorwas turned off may be encoded into the sensor in memory 236. When theoximeter monitor is turned back on again, the monitor can display thedate and time that it was last turned on and off. It may be desirablefor medical personnel to know the last time that patient's vital signswere monitored by the oximeter. The sensor memory can also store anindication of a disconnection of the sensor from the monitor. This dataallows the clinician or medical personnel to delineate the eventsretrieved from the sensor memory.

The oximeter monitor instrument may also write the alarm limits usedwith a particular patient into memory chip 236. Alarm limits are valuesthat represent maximum or minimum values of patient trending datatracked by the oximeter (such as blood oxygen saturation, pulse rate,pulse amplitude, etc.) that will trigger an alarm, because they areconsidered to be dangerous levels. The alarm limit values may be encodedin memory 236 by the manufacturer or by a clinician through the oximetermonitor prior to operation.

The oximeter monitor periodically checks the patient's monitoredtrending data against the alarm limit values. When one of the monitoredpatient parameters reaches the alarm limit value stored in memory 236,the oximeter monitor triggers an alarm which alerts medical personnelthat a problem may exist. Present embodiments also allowpatient-specific alarm values to be set by medical personnel through theoximeter and stored in memory 236 so that as the patient moves frommonitor-to-monitor (while the sensor stays with the patient), theappropriate alarm limits need not be reset each time on the new monitor.Instead, the alarm limits only need to be programmed once, or at a latertime, whenever the clinician adjusts alarm limits.

One or more of the patient trending data including blood oxygensaturation, pulse rate, and pulse amplitude can be written to memory 236along with a time of occurrence whenever an alarm threshold is crossed.Additional information, such as the readings for a predetermined timeprior to an alarm occurrence can also be stored, and/or periodic valuesduring the alarm breach can also be stored in memory 236.

Currently sensors are placed on patients at one hospital site and staywith the patient from hospital site-to-site. It would therefore bedesirable to have a patient identification code (patient ID) such as aunique number carried along in the sensor so that the record keeping,which occurs at each site, can link the recorded information with thepatient. Without a patient ID stored in the sensor itself, the trackinghas to be done manually.

Thus, in a further embodiment, the oximeter monitor can store a patientID in memory 236 of sensor 130. The oximeter has an input device such asa keyboard, touch screen, or scanner that allows a patient ID to beentered and reentered into the oximeter so that it can be stored insensor memory 236. With patient trending information being stored inmemory 236 of the sensor 130 as discussed above, it is also desirable tohave the patient ID stored in memory 236 so that as the patient goesfrom hospital location to location, the new location's staff can verifythat old trending information stored in memory 236 was indeed obtainedfrom that particular patient. Medical personnel can check that thepatient ID stored in sensor 130 matches the patient ID on the patient'schart and other paper documentation to verify that these medical recordscorrespond to the correct patient. If desired, the oximeter sensor canbe interfaced with a hospital computer network that maintains a databaseof patient ID numbers to verify the identity of the patient and toobtain medical records and other information for the patient stored onhospital databases. The patient ID stored in memory 236 providesassurance that any data read from memory 236 of the sensor is correlatedwith the patient they are receiving.

The pulse amplitude of the measured photoplethysmogram is an indirectmeasure of blood perfusion (flow) in the local tissue, changes in bloodpressure, vascular tone, vasoconstriction or dilation, for example, allhave an effect on the pulsatile signal strength observed with a pulseoximeter. The measured modulation, or other measurement of perfusion,can be stored in memory 236 for patient trending purposes. The oximetercan compare current modulation and perfusion data with older data frommemory 236 to determine patient trends over time. The patient's pulseamplitude deteriorating over time may reflect a serious condition thatdemands attention. Therefore, it is desirable to store and monitorchanges in a patient's perfusion over time. Also, a maximum or minimumperfusion limit may be stored in memory 236 that represents the maximumor minimum value that the patient's measured perfusion can reach beforethe sensor needs to be moved, repositioned, or adjusted in some otherway. The oximeter can trigger a warning signal or light when a perfusionlimit has been reached or a significant change has occurred.

The sensor memory can also include a field that indicates when thesensor memory is full. The information in this field can be provided tothe monitor to direct the monitor to cease sending data to the sensormemory. The information in this field can be prominently displayed bythe monitor to notify the clinician or medical personnel. Also, inresponse, the monitor can generate an alarm (i.e., blinking light or anaudio alarm, or both) to draw the attention of the clinician to theoperating state of the sensor.

The sensor memory can also store data for various physiologicalcharacteristics such as, for example, heart rate, blood pressure,temperature, and so on. For example, the sensor memory can be used tostore NIBP, IBP, and ECG waveforms. Moreover, as memory costs continueto fall and larger memories become available, more complex physiologicalparameters can be measured and stored.

Additionally, information about the monitor can be stored or embeddedalong with the physiological data. This additional information mayinclude, for example, the serial number of the monitor to which thesensor couples, the sensor connect/disconnect times, monitordiagnostics, and others. This information would allow the clinician toaccess historical information on the instrument as well as thephysiological data, which might be useful, for example, in productliability and malpractice litigation or in troubleshooting instrumentperformance questions.

The memory 236 associated with the sensor can be physically located in avariety of places. First, it can be located on the body of the sensor,in a vicinity of the photodetector, LEDs, or other sensor components.Or, the memory can be in the sensor connecting cable 124 or the sensorconnecting plug 122, or in an adapter module that connects to a front ofan oximeter, to an oximeter cable, or to a sensor plug or cable. Whetheror not the memory 236 is in the sensor body, sensor cable 124, sensorplug 122, or adapter module, it is always “associated with” the sensorsince the memory travels with the sensor and patient when the patient ismoved and the sensor is disconnected from the monitor.

The monitor and sensor can be configured such that the data is providedto and stored in the memory 236 automatically and continuously. Themonitor and sensor also employ automated event recording, such that themonitor in response to an oxygen desaturation event transfers some ofthe physiological data to the sensor memory. Alternatively, particularlywhen a size of the memory is small, the monitor can require a user firstcommand the monitor (e.g., by activating a control knob) to send data tothe memory 236. In this case, valuable storage space in the memory 236will only be used (and consumed) when the patient being monitored isbelieved to be relatively unstable by the caregiver and when it isbelieved storage of the historical data for later retrieval may beparticularly desirable.

As noted above, the techniques can be used to store and provide variousphysiological data including, but not limited to, blood oxygensaturation, heart rate, temperature, and blood pressure data. Forclarity, the invention is described in the context of the storage andretrieval of blood oxygen saturation (SpO₂) data. Based on the receivedsignals representative of the intensity of the light detected byphoto-detector 240, processor 262 estimates oxygen saturation usingalgorithms that are known in the art.

The saturation data for a particular patient is processed by the monitorattached to the sensor, and the processed data is provided to the sensorfor storage in the sensor memory. The selection of the sensor memory isdependent on numerous factors including cost, the amount of data thatneeds to be stored for a particular application, the amount ofachievable data compression (if compression is used), the physicaldimensions, and so on. For oxygen saturation, storage of approximatelyone to seven days of historical data is adequate for many applications.

In an embodiment, to reduce the amount of data to be stored in thesensor memory, the physiological data is compressed before storage. Inan embodiment, the compression is performed by facilities located withinthe monitor. Alternatively, the compression encoding circuit can be onthe sensor itself. The monitor further includes facilities to decompressthe data later retrieved from the sensor memory. Compression allows forthe use of a smaller-sized memory in the sensor. This is particularlyadvantageous in the case of single patient use disposable sensors whichare typically disposed after use on a patient. Compression also allowsmore data to be stored into a memory of a given size. The ability tostore a large amount of data is important for many diagnosticapplications that require data collected over hours or days.

The compression scheme can be designed to take advantage of knowncharacteristics of the physiological data being stored. For example, itis known that oxygen saturation generally does not change rapidly. Thischaracteristic can be exploited to achieve significant compression, asdescribed below.

For arterial oxygen saturation data, one compression scheme is based onthe realization that saturation data generally exists in two dimensions,time and absolute saturation value. Saturation sample time is about onehertz normally, assuming one saturation value is determined by a monitorfor each patient heartbeat. The number of possible saturation valuesgenerally corresponds to 101 possibilities, which assumes eachpossibility corresponds to an integer saturation percentage which mustlie somewhere between 0% to 100%. A goal is to use a compression codingtechnique which reduces the scope of these two dimensions, which againare 1 Hz by 101 saturation values, to a smaller region, e.g., 0.1 Hz by8 saturation values, for example, and yet retain the usefulness of theinformation. According to one embodiment, Huffman compression is usedwith run length encoding, or alternatively or additionally differentialencoding.

Run length encoding simply means that if the same saturation value is tobe repetitively transmitted, rather than repetitively transmitting thisvalue, it can be more efficient to count the number of consecutive timesthe value occurs, and to instead transmit the value and the number oftimes it is repeated. Differential encoding, though similar, isdifferent in that a difference between consecutive saturation values tobe transmitted is calculated, and along with the original saturationvalue one transmits the differential between adjacent values and thenumber of times an identical differential occurs. There are many knownpermutations of these two types of common compression techniques knownin the art.

As noted above, if saturation percentages were to be reduced to a groupof eight values, any single value would correspond to a range ofsaturation percentages. For example, ten values could be selected, witheach value corresponding to an incremental increase of 10 saturationpercentage points. A disadvantage of such a grouping is that a patientwho has a saturation which is toggling between groups by only onesaturation percentage, e.g., toggling between 89% and 90%, can require agreat deal of memory to record this information since the advantages ofrun length encoding are greatly diminished if the identity of the groupis repetitively changing.

Accordingly, it has been determined that it may be desirable to utilizesaturation values which have saturation percentages which overlap. Forexample, eight saturation values could be as follows:

Saturation Value # Saturation Range (%) 1  85-100 2 80-90 3 75-84 470-79 5 65-74 6 60-69 7  1-64 8 0

Since these saturation values have saturation percentages which overlap,toggling at a boundary between values 1 and 2 is prevented for smallsaturation, changes. For example value 2 has previously been storedbased on a saturation of 90%, and the saturation moves to 91%, thesaturation range will now correspond to value 1, and this value will bemaintained until the patient saturation falls below 85%. Hence, withoverlapping values or groups, it is readily apparent the advantages ofrun length encoding will be more readily achieved for all patientsexcept the most unstable whose saturation varies by relatively largeamounts in relatively short periods of time.

Turning to the other dimension involved with saturation, specificallysaturation sampling frequency, for a sampling rate of 0.1 Hz, a singlesaturation value can be determined for a patient based upon theiraverage saturation every 10 seconds. Lower sampling frequencies could beused which generate less data, which consume less memory, but adisadvantage is that correspondingly less information will be stored inthe memory. Conversely, higher sampling frequencies could be used whichresult in more information being stored in the memory, with acorresponding disadvantage that more memory is required.

Regardless of the saturation sampling frequency, and the number ofsaturation values or groups which are used, further compression can beachieved by overlaying Huffman encoding techniques, which simply meansthat if a mean value to be transmitted is relatively high, such as 95,this value can be normalized to 0 using Huffman encoding techniques withthe result that fewer bits are required to transmit relatively largenumbers by such normalization.

Several compression embodiments have been described for oxygensaturation data. Although present embodiments can be practiced withoutthe use of compression, additional capabilities are provided by thejudicious use of compression. As used herein, compression includes anyprocessing that alters, however slightly, the original form of thephysiological data as they are generated (in the nominal manner) by themonitor. Other compression schemes can also be used and are within thescope of the invention. Of course, no compression could be used.

As noted above, in a specific embodiment, the sensor memory isimplemented as a write-once memory device. A field in the sensor memorycan be set when the sensor is reprocessed so that the monitor candetermine that it is coupled to a reprocessed sensor. The monitor canuse the information in this field to disable the display of thehistorical data (for example, if the memory is write once and relativelyfull). Alternatively, if the memory is erasable, a field for storinghistorical physiological data could be erased during sensorreprocessing.

Disabling the data display may be preferable in some applications toensure the integrity of the collected data. For a memory device that canbe written once and has a fixed memory size, it may not be possible todetermine where the “old” data came from or how much memory may still beavailable on a reprocessed sensor. Moreover, it is highly desirable toavoid having data from an old patient being displayed and potentiallymistaken as valid data for the patient to which the sensor couples.Since it is not easy to control or determine the amount of availableunwritten memory after a use, which can vary from zero to the fullamount, inconsistency and potential customer dissatisfaction may resultfrom using a sensor having widely varying amounts of available memory.By not displaying data from reprocessed sensors, these potentialproblems are avoided.

Embodiments may provide advantages not available in conventionalmonitors and sensors. For example, certain present embodiments allow formonitoring of a patient in transit who may be connected to two or moremonitors over a period of time. One such situation is a patient who istransported in an ambulance to an emergency room and later transferredto an intensive care unit. This technique is especially beneficial inthis application since this particular patient is more likely to be inneed of close monitoring and recent historical physiologicalinformation.

Present embodiments can also be used to document physiologicalcharacteristics. For example, for a patient in home care who requiresoxygen, documentation of oxygen saturation is typically needed. In thiscase, the sensor of the invention can be used to store saturation datafor the patient over a predetermined time period (i.e., one week). Atthe end of this period, the caregiver can simply remove the sensor andsend it away as documentation of the patient's saturation. Thesetechniques can also be used to collect data for other applications suchas, for example, sleep diagnostics, de-saturation, and so on.

The sensor in accordance with present embodiments has been described foruse in combination with a monitor that performs the signal processing ofthe detected signal and compression of the processed data. In anotherembodiment, the sensor of the invention includes the facility to process(and compress, if necessary or desirable) the detected signal. Thisembodiment advantageously allows for independent operation of the sensorwithout support from a monitor. The data stored within the sensor can beprovided to a monitor for display. The amount of signal processing andcompression that can be achieved by circuitry within the sensor is onlylimited by the available technology, which inevitably improves overtime. In the near term, physiological data that does not requireextensive signal processing and compression (e.g., temperature, peakamplitude in a waveform, heart rate, and so on) can be collected andstored by the sensor.

FIG. 3 is a block diagram of a pulse oximeter system incorporating acalibration memory element 302 in accordance with embodiments of thepresent invention. In one embodiment, memory element 302 is a two-leadsemiconductor digital memory chip. The calibration element 302 is partof a sensor 304 which also includes red and infrared LEDs 306, alongwith a detector 308. If desired, LEDs 304 may be replaced with otherlight emitting elements such as lasers.

The oximeter includes read circuit 310, drive circuit 312, look-uptables 314 and 316, controller 318, amplifier 320, filter 322, andanalog-to-digital converter 324. Read circuit 310 is provided forreading multiple coded values across the two leads 326, 328 connected tocalibration element 302. One value is provided to a look-up table 314 todetermine appropriate wavelength dependent coefficients for the oxygencalculation. The other value(s) are then provided to another look uptable(s) 316 which provides input (e.g., coefficients) to othercalculations performed by controller 318. These additional calculationsmay enhance the performance and/or safety of the system. Controller 318provides signals to a drive circuit 312, to control the amount of drivecurrent provided to LEDs 306.

Detector 308 is connected through the amplifier 320 and the filter 322to the A/D converter 324. This forms a feedback path used by controller318 to adjust the drive current to optimize the intensity range of thesignal received. For proper operation the signal must be within theanalog range of the circuits employed. The signal should also be wellwithin the range of A/D converter 324. For example, one rule that may beapplied is to adjust LED drives and amplifier gains so that both red andIR signals fall between 40% and 80% of full scale reading of converter324. This requires correct and independent settings for both the red andinfrared LEDs.

Part II

Embodiments of the present technique include the following:

Sensor Model ID

Encoded text of the specific model of sensor would allow the instrumentto display a text string indicating what sensor is being used, e.g.“Nellcor OXISENSOR II D-25” or “Adult Digit Sensor” or “Agilent N-25”.Alternately, a sensor code could be stored that points to a lookup tableof display text. Encoding sensor model ID could also be used toaccommodate sensor-specific operating parameters such as LED drivecurrents or “sensor off” characteristics (as an alternative toprogramming the value of drive current or “off” characteristicsthemselves).

Sensor Model—Specific Information

Coefficients for Taylor's Series Calibration Curves

The sensor may store a general polynomial curve. Other families ofpolynomials, such as Tchebyschev polynomials, could be used as well.This may also pertain to other calibration information, such astemperature calibration and force transducer calibration. This allowsnew sensor types (such as a sensor with an offset emitter and detector).

Sensor Adjustment/Re-Application Light Levels

Sensor Off Light Levels

Under normal operating conditions, photosignals coming from the sensorLEDs generally fall within a certain range. When a sensor is removedfrom a patient, or falls off on its own, the photosignal usuallychanges. This is particularly true for the reusable clip-style sensor,since in their normal disconnected state, the LEDs shine directly ontothe photodetector unimpeded by, for example, tissue. By programming a“threshold photocurrent” into the memory chip, reliable detection of a“sensor is off the patient” condition can be accomplished (in thisexample, exceeding a certain detected light level is a sure sign thesensor is not on a finger or other opposed site). For certain othersensors, a low light level may be indicative of the sensor being off (anadhesive sensor, for example, lays flat when in its natural state, solittle LED light may reach the detector). Encoding an expected range oflight levels for the specific model of sensor being used allows enhanceddetection of when the sensor is improperly placed or has been removed.

Temperature at which to Switch to Storm Algorithm

The STORM algorithm here refers to the sensors designed to be used where“motion provides the signal”, i.e., the cardiac pulse need not bepresent or discernible in order for the oximeter to provide SpO₂ values.Instead, the red and IR waveforms resulting from the motion itself areused for determining the arterial saturation. This feature is possiblefor tissue beds that are well “arterialized” (a large supply of arterialblood relative to the metabolic needs of the tissue) resulting in asmall aterio-venous saturation difference, as well as other signalcharacteristics that are not germane to this discussion. It has beenobserved that the necessary degree of arterialization correlates well tobeing “well perfused” at the tissue site, which itself correlates wellto the tissue bed being warm. Thus by monitoring the temperature of theskin at the sensor site, and by knowing a value of temperature(programmed into the memory chip) at which the “motion-is-signal”algorithm can be utilized for the specific sensor design being used,improved reading accuracy through motion can be better accomplished.

Additional Information on Use of Pins

Contact Switch—Sensor Off

Similar to the contact electrodes of the Nellcor FS-14 fetal sensor, anextrinsic probe of skin contact can be used to indicate whether thesensor is in adequate contact to the patient. This extrinsic probe couldbe accomplished, for example, through an impedance measurement acrosstwo electrodes, a force or pressure switch that is sensitive to whetheradequate force or pressure is present in the sensor placement, orthrough other means. Dedicated sensor connector pins, or pin-sharing,could be used to accomplish this additional measure of sensor-patientcontact.

Chemical Sensor for EtO Cycles

An electro-chemical or thermal device that senses and stores to memorythe number of exposures (zero, once, or potentially more than once orthe actual number) to sterilization cycles could be used to capture thehistory of the sensor. Excessive exposure to sterilization cyclesdegrades a number of components in the sensor, and can affect itsperformance. A sensor exceeding a certain number of exposures couldcause a display to indicate the sensor needs to be replaced.

Sensor Expiration

This need not be a separate device, but the memory could contain a dateafter which time the sterilization can no longer be certified as beingeffective. Sterilization can be sensed and the date recordedautomatically by the sensor itself.

Sensor Expiration Date/Sensor Parking: Meter

At the time of manufacture, the expiration date of the sensor may bewritten into the memory chip. The memory-enabled instrument would thendo something with this knowledge (e.g., indicate “expired sensor”, orrefuse to function if expired). Alternately, the elapsed time of sensorusage could be tracked in the memory chip (written to it by theinstrument) and the sensor would “expire” after a memory programmedmaximum (greater for reusable sensors than for single-use sensors).

Auto Shut-Off

After sensor expiration, the instrument may refuse to function with thissensor and would indicate that a fresh sensor is needed. Furthermore,the sensor could be disabled by running a high current through it, or byother means.

Warranty Date

Similar to the expiration date, the date of expiration of the sensorwarranty could be written into the memory chip (e.g., 2, or 6, or 12,etc. months from the date of 10 manufacture or the date of first use).The instrument would give some indication of this as appropriate.

Patient Specific Information (Written to Sensor from Monitor)

Trending and/or data logging of patient monitoring parameters may bestored in the memory of the memory chip. As the patient and sensortravel from ward-to-ward of the hospital, and consequently plug intodifferent oximeters, the patient-specific data could be displayed as itis contained in the patient's dedicated sensor. Examples of the type ofdata are given below:

Trending

Low High Sat

The lowest and/or highest SpO₂ value during the monitored time, or thelowest/highest values over the past specified monitoring time (e.g., 2hours, 1 day, etc.)

Duration of Monitoring

How long has the patient been monitored by the pulse oximeter? (elapsedtime counter).

Beginning and End of Monitoring

Clock time of when the monitor was turned on and off.

Pre-Set Alarm Limits

The alarm limits used with this patient become written to the memorychip by the instrument. This allows patient-specific alarm values to beset and memorized so that as the patient moves from monitor-to monitor(the sensor staying with the patient), the appropriate alarm limits neednot be reset each time on the new monitor. Instead, this only needs tohappen once, or whenever the clinician adjusts alarm limits.

Changeable Key

Data encryption utilizes private and/or public keys to scramble the datawritten to the memory chip and later decipher the data so that onlyauthorized devices are supported. To further prevent the use with amonitor that is not certified to provide correct results, the sensormanufacturing system could periodically change the private and/or publickeys. The change in the key could be communicated to the instrument viathe memory chip in encrypted form. The purpose of this feature is toelevate the level of security in the memory system.

Monitor Code Upgrades from Modem or Sensor

Distributing code updates in memory. Whenever an oximeter notes that acode update field is present in the sensor, it would check whether theproposed update had previously been installed, and (if not) whether anyindicated prerequisites were present (e.g. a code patch might notfunction properly in the absence of a previously-circulated patch). Ifappropriate conditions are met, the code upgrade would be installed. Ifprerequisites are missing, a message would be displayed to the user,telling him how to obtain the prerequisites (e.g. call Nellcor).

Black Box Encoder (Patient History, Serial Number of Box. Etc.)

Use the memory as a general-purpose carrier of patient data, coveringnot just oximetry but a lot of other information about the patient.

Optical Efficiency Correction

If it is desirable to know where a particular patient lies in COP space,it is useful to know the inherent brightness of LEDs, sensitivity ofdetector, and anything else about the particular sensor assembly (e.g.bandage color and alignment) that will affect the amount of light whichthe sensor receives. Given that information, a measure-of the patient'soptical transmissivity may be computed for each LED wavelength, whichdepends almost entirely on the properties of the patient. Signal tonoise ratio of the oximeter is probably determined by the size of thedetected signal, not by the transmissivity of the patient alone. Thiscould take advantage of DC transmissivity of the tissue to improve theaccuracy of pulse oximetry.

Another reason for recording LED and detector parameters in the sensormemory is to provide a basis for later research on the drift of theseparameters due to various environmental conditions which the sensorexperiences. Parameters of interest include not only LED power anddetector sensitivity, but also LED wavelengths, FWHM, and secondaryemission level.

Pigment Adjustment Feature

For some types of sensors, the accuracy of the sensor may be differentfor patients with different skin color. The sensitivity of accuracy toskin color may depend on sensor model. The sensor might contain asensitivity index, indicating how large an adjustment in readings shouldbe made as a function of skin color. Skin color might be obtained byuser entry of the data (e.g. menu selection). Another option would befor the sensor to measure skin color. One way to achieve the latteroption would be to provide transmission sensors with auxiliary detectorfor “reflected” light. In combination with the optical efficiencyinformation noted above, the signal levels reported by the auxiliarydetector would sense to what extent the patient's skin was affecting redand IR pathlengths differently, and hence to what extent readings neededto be corrected.

Accelerometer on Chip

This might be used in a scheme in which the memory chip was on thebandage, not in the connector. This combines a MEMS accelerometer withany of several different chips that might usefully be placed in thesensor head, local digitizing chip, preamp chip, memory chip.

Accelerometer data may be used to warn of the presence of motion (inwhich case special algorithms may be called into play or oximetry may besuspended), or actually to help correct for motion (to the extent towhich we can produce algorithms which can predict physio-optic effectsof known motion).

Optical Shunt

The amount of optical shunting could be measured for each sensor, orfamily of sensors. The value would be stored in the sensory memory forthe monitor to read and use to adjust the processing coefficients.

Monitor Chip Temperature

The temperature of a red LED, in particular, affects its principalwavelength, which affects calibration. For one class of LEDs, thewavelength shifts by about 0.14 nm/C. The memory chip might containcircuitry capable of monitoring a thermistor or thermocouple, or thememory chip could be mounted in proximity to the LED (e.g. under it), sothat it could sense directly the temperature of the LED, and provide acalibration correction accordingly.

Monitor Ambient Temperature

This might be used, e.g., in overseeing the operation of a warmed earsensor. There is a thermal cutout in the control system of the WES, thatcauses operation to terminate if the sensor goes over a certaintemperature. This is a component for protecting the patient againstburns. If the reason for a high sensor temperature is that theenvironment is warm, it could be quite acceptable to continue oximetry,even though warmer operation would be shut down. In the absence ofknowledge about environmental temperature, a high temperature readingmight have to be assumed to mean that something was wrong with thesensor, in which case all operation might have to cease. Anenvironmental temperature sensor in the plug could help to tell whichrule to apply. Again, the memory chip could record the calibration ofwhatever device was used for thermometry.

A passive component on the memory chip could be the thermometric sensor,and a resistance or voltage measuring device in the instrument couldread out that sensor. Thus, ambient temperature sensing might notrequire that large changes be made in the memory chip.

Temperature Amplifier/Detector

In illuminating the skin for the purpose of making oxygen saturationmeasurements, some heat is generated by the LED emitters. Tests havebeen done to establish the maximum safe current for the LED drive whichwill assure that the skin temperature stays within a safe value for theworst case sensor/patient conditions. This means that in all cases thesensor will be operated at cooler than the maximum temperature but inmost cases well below the maximum temperature.

To establish the optimum signal for the measurement, it is desirable todrive the LEDs with higher current than is imposed by the abovelimitations. The temperature amplifier/detector would allow the LEDs tobe driven to a level that still results in a safe temperature bymonitoring the temperature, yet in many cases allow more drive current,and therefore higher signals, which could give better readings.

The inexpensive thermistor devices that could be used in thisapplication are characterized to allow the measurement to be accurate.These characterization values could be stored in the sensor where thethermistor is located. While in operation, the oximeter would be able toread the characterization values from the sensor, measure the resistanceof the thermistor, and calculate accurately the temperature of the skinsurface where the thermistor is located. This would keep the patientsafe from burns and still provide the best signal available.

RCAL Resistance Built into Chip

In legacy oximetry sensors there is a resistor which is selected andinstalled in the sensor connector, to correspond to the wavelength ofthe red LED. The wavelength difference from LED to LED has an impact onthe calibration of the saturation measurement, if not compensated for.The oximeter will read the value of resistance and adjust itscalculation accordingly.

When adding the memory chip, memory compatible oximeters will be able toobtain the necessary calibration coefficients from the memory chip butthe legacy instruments will still need a calibration resistor value. Ifthe resistance were built-in to the chip and trimmed or selected atmanufacture then only one device would need to be installed in thesensor connector. That would reduce the overall cost, yet keep thesensor compatible with both the legacy instruments and the new memorycompatible instruments.

Secondary Emission Measurement

The oximeter is measuring the relative transmission of the red andinfrared light through the tissue. LEDs have a characteristic calledsecondary emission which is indicative of the amount of light, atwavelengths other than the primary wavelength, that is being emitted.This characteristic will change the calibration of the device if notcompensated for. It is possible to make an oximeter that will functionwithin calibration if the secondary emission is known and compensatedfor. If the LED were characterized during manufacture and then thesecondary emission values entered into the memory chip, the oximeterwould be able to read those values and compensate for them so that thesensor was used properly. This would increase the range of LEDs thatcould be used for oximetry, reduce cost and provide better calibrationacross a wider range of LED emitters.

Patient ID (Potentially as Tracking Device, Archiving Patient History,Etc.)

Currently sensors are placed on patients at one hospital site and staywith the patient from hospital site-to-site. It would be helpful to havethe patient ID carried along in the sensor so that the record keeping,which occurs at each site, would be able to link the recordedinformation with the patient. Without patient ID, the tracking has to bedone manually. With trend information being stored in the sensor it alsowould be desirable to have the patient ID included so that as thepatient went from location to location, the new location's staff couldverify old information and be assured that it correlated with thepatient they were receiving.

Encode Contact Resistance (e.g. for Oxicliq)

When making measurements of the resistance that is placed in the sensorfor calibration information purposes, one of the factors that caninfluence that measurement is the contact resistance of the connectorsthat are between the oximeter and the resistor itself. In order tocompensate for connectors that are significant in their impact on themeasure, one could encode the contact resistance of the connector andsubtract that algorithmically from the measured resistance to get a moreaccurate measurement of the resistance of the calibration resistor. Thiswould enhance the accuracy with which the resistance measurement is madeand, therefore, make the instrument less prone to inaccuracies insaturation calculation and display.

Measure Capacitance to Balance Common Mode Rejection

One of the interfering noise sources that plagues oximetry is that ofcommon mode noise. This can come from the surrounding electricalenvironment. Other instruments, lighters, drills etc. can produceelectrical fields that can couple into the cable between the patient andthe oximeter. Once coupled-in, they can make measurements moredifficult, less accurate, or not possible, depending on the severity ofthe noise. To help reduce this common mode noise, differentialamplifiers are used for amplifying the signal from the sensor. Theseamplifiers amplify only the difference between two signal wires. Thus,if the common mode signal is coupled exactly the same into both wires,the amplifier will not amplify it because the same signal is present onboth wires.

If the two wires have different coupling to their electricalenvironment, then they will present different signals, and thedifference will be amplified as if it were a signal. One component thatinfluences this coupling is the capacitance of the lines to the outsideworld. This is affected by the manufacture of the cable, materials,twists in the wire, etc. If one measured the cables during manufactureand then stored that information in the memory chip, it could be readwhen the oximeter is operating. Once the capacitances for the two wiresto the shield are known, the instrument could be provided with a tunablecapacitance device that would then balance the two lines again and makethe noise coupling to the lines better matched. This would reduce theamount of susceptibility to the external noise that got coupled into thepatient cable. Reduced noise results in better measurements or theability to make measurements at all on some patients.

Fiber-Optic Infrared Wavelength Shift

The relative wavelengths of the red and infrared light that is used tomake the measurement in oximetry are important to know so thatcalibration can be maintained. In traditional LED oximetry, the LEDsources are at the skin so that whatever wavelength is emitted is whatis sensed by the photodiode that receives the light. The red LED is theonly one that we need to characterize for accurate saturationmeasurements to be realized. The saturation is less sensitive to the IRwavelength as long as it stays fixed in the acceptable range that hasbeen specified for the IR LEDs.

When using plastic fibers for transmission of the light, there is awavelength dependent absorption caused by the fiber. This has the effectof altering the apparent center wavelength of the IR source, which canaffect calibration of the unit. By characterizing the fiber for itsshift, one could then provide the proper compensation in the algorithmthat calculated the saturation. This would restore the accuracy thatwould otherwise be lost in fiber transmission of the light.

Inform Monitor of Extra LEDs

There are limitations on the number and type of blood constituents thatcan be sensed using the two conventional LED wavelengths of theoximeter. The accuracy of the oximetry measurement can also be improvedby using different wavelengths at different saturation ranges. Ananalysis unit could be developed that would utilize either or both ofthese features. To do this, it would be able to drive additional LEDs.The additional LEDs could be driven along with the traditional ones orseparately. The oximeter (or additional constituent measurement unit)would provide the capability to calculate values for these otherwavelengths, and the sensor would provide the additional information toallow the oximeter to make that calculation. These could be stored inthe memory.

Active Ambient Light Measurement

One of the problems with oximetry sensors is the interference caused byambient light in the environment. This can be made worse when a sensorcomes loose or when the ambient light is extremely high in value. Bycharacterizing the sensor, one could know what level of ambient lightcould be expected or tolerated, and give a warning to the user when thelevel has been exceeded. This would give them the opportunity to adjustthe sensor, the light, or both to affect an improvement in theperformance of the oximeter.

Active Pressure Adjustment for Modulation Enhancement

The stronger the pulsatile signal, the better the chances are ofmeasuring the saturation accurately. One way to enhance the modulationpercentage is to apply pressure in the range of the median pulsatilepressure. If this were implemented, one could use relatively low costtransducers and supply calibration coefficients in the memory to allowaccurate pressure readings to be made. The memory could also contain thepressure settings and/or expected modulation enhancement capability todetermine effectiveness of the pressure enhancement.

Measure Perfusion

The amount of perfusion affects the amount of modulation, and thus theAC signal. This affects both the percentage of modulation vs. the DCamount, and the absolute size of the modulation. The measuredmodulation, or other measurement of perfusion, can be stored in memoryfor trending or setting limits on acceptable perfusion before movementor other adjustment of the sensor is required.

Keep Track of Last Time Sensor Moved or Disconnected

The time of movement or disconnecting of the sensor could be writteninto the memory. Disconnecting can be detected from the interruption ofthe signal to the monitor. Moving can be detected by a sensor offdetection, and a subsequent sensor on detection. Alternately, aggressivemovement could be detected and interpreted as moving of the sensor, or acombination with a sensor off detection could be used.

Identify Private Label Sensors

A code can be stored in the sensor memory identifying the sensormanufacturer. This code can be read and used to indicate operabilitywith monitors of other manufacturers, or to indicate any conversionalgorithm that may be needed for a signal from a sensor to be used by amonitor from a different manufacturer. The code can also be used toallow only supported features to be used.

Measure Sensor Wetness

A moisture sensor or impedance sensor can detect the amount of wetnessof the sensor. This can be used for different purposes and can be storedin the sensor memory for trending or monitoring. To determine sensormalfunction, the sensor can be disabled if the wetness exceeds athreshold, which could be stored in the sensor memory. Some sensors maynot provide for isolation of the patient from the electronics forexcessive wetness. The maximum allowable wetness could be stored in thesensor memory.

Sensor Isolation Indicator

The sensor memory could identify that the sensor provides isolation, sowetness is not a concern. Alternately, it could indicate that isolationis not provided by the sensor, or a limited amount of isolation isprovided.

Low Power Mode Identifier (Sensor Tells Oximeter to Sleep or Wake Up)

A portable battery-powered monitor can power down when the saturation isat a good level, and the patient is stable. Minimal circuitry in thesensor could be used to do sufficient processing to tell the monitorwhen to wake up.

Battery to Run Digital Chip

A battery can be included in the sensor for a wireless connection to amonitor. Alternately, a battery could be used to continue to send datawhen the monitor is powered down.

Motion Generator (“Thumper”)

The sensor can include a cuff (which inflates and deflates) or othermechanical mechanism for inducting motion to get a signal or forinducing pulsitile blood flow to improve the signal.

Sensor Force Indicator (e.g., Too Tight)

A transducer can measure the amount of force on the sensor. This can becompared to a maximum value stored in the sensor memory to determine ifthe sensor is on too tight. The tightness can also be recorded andmonitored. For example, a patient could swell, and this could bedetermined from the trend information and provided as information to aclinician on a display.

Force Transducer Calibration to Get Pressure

A calibration value can be stored in the sensor memory for converting aforce measurement into a pressure measurement. A force transducer canthen be used to measure pressure.

Number of Wavelengths

The sensor memory can store an indication of the number of wavelengthsused in the sensor, and could store the wavelengths themselves.

Drive Information

The sensor memory can store information about when to drive which LEDs.They could all be driven at once, or a subset could be driven, forexample.

Display for Additional Wavelengths

The memory can store information about what parameters are to beanalyzed and displayed when the extra wavelengths are used. Oxygensaturation may be displayed when 2 wavelengths are used, whileadditional information could be displayed when an extra wavelength ormore are used (Hct, COHb, etc.).

Recycling Times

Each time a sensor is recycled (sterilized and reconstructed), a numberin the sensor memory can be incremented. This can be used to preventoperation of the sensor if it has been recycled more than the allowednumber of times (e.g., 3 times).

While the present invention has been described herein with reference toparticular embodiments thereof, a latitude of modification, variouschanges and substitutions are intended in the foregoing disclosure, andit will be appreciated that in some instances some features of theinvention will be employed without a corresponding use of other featureswithout departing from the scope of the invention as set forth.Therefore, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope and spirit of the present invention.It is intended that the invention not be limited to the particularembodiments disclosed, but that the invention will include allembodiments and equivalents falling within the scope of the claims.

1. An oximeter sensor, comprising: a light emitting element configuredto emit light; a light detector configured to detect the light; aresistor having a resistance value that is detectable by a monitor; anda memory storing a contact resistance value of a connector that isdisposed between the monitor and the resistor when the monitor and theoximeter sensor are communicatively coupled, the contact resistancevalue being readable by the monitor to be used in a correction mechanismof the monitor to correct the resistance value based on an algorithmthat incorporates the resistance value and the contact resistance value.2. The oximeter sensor of claim 1, wherein the resistance valuecorresponds to a calibration resistance value that facilitates obtainingcalibration coefficients in the monitor.
 3. A method of manufacturing anoximeter sensor, comprising: providing a light emitting element withinthe oximeter sensor, the light emitting element configured to projectlight; providing a light detector within the oximeter sensor, the lightdetector configured to detect the light; providing a resistor within theoximeter sensor, wherein the resistor includes a resistance value thatis detectable by a monitor; and providing a memory within the oximetersensor, wherein the memory is storing a contact resistance value of aconnector that is disposed between the monitor and the resistor when themonitor and the oximeter sensor are communicatively coupled, the contactresistance value being readable by the monitor to be used in acorrection mechanism of the monitor to correct the resistance valuebased on an algorithm that incorporates the resistance value and thecontact resistance value.
 4. The method of claim 3, comprising storing acalibration resistance value in the memory, the calibration resistancevalue facilitating obtaining calibration coefficients in the monitor. 5.An oximeter system, comprising: an oximeter sensor, comprising: a lightemitting element configured to emit light; a light detector configuredto detect the light; a resistor having a resistance value; and a memorystoring a contact resistance value of a connector that providescommunicative access to the resistor; and an oximeter monitor,comprising: a drive circuit configured to provide signals to circuitryof the oximeter sensor; a read circuit configured to read the resistancevalue of the resistor and the contact resistance value from the memory;and a correction mechanism configured to correct the resistance valuebased on an algorithm that incorporates the resistance value and thecontact resistance value.
 6. An oximeter monitor, comprising: a drivecircuit configured to provide signals to circuitry of an oximetersensor; a first read circuit configured to read a resistance value of aresistor of the oximeter sensor; a second read circuit configured toread a contact resistance value from a memory of the oximeter sensor;and a correction mechanism configured to correct the resistance valuebased on an algorithm that incorporates the resistance value and thecontact resistance value.
 7. The monitor of claim 6, comprising amonitor memory storing a calibration table of resistance values andcalibration coefficients.
 8. A method of monitor operation, comprising:providing signals to circuitry of an oximeter sensor; reading aresistance value of a resistor of the oximeter sensor; reading a contactresistance value from a memory of the oximeter sensor; and correctingthe resistance value based on an algorithm that incorporates theresistance value and the contact resistance value.
 9. The method ofclaim 8, comprising storing a calibration table of resistance values andcalibration coefficients.
 10. A method of operating an oximeter sensor,comprising: projecting light from a light emitting element; detectingthe light with a light detector; and producing a resistance value with aresistor, the resistance value being detectable by a monitor; andproviding the monitor access to a value stored in a sensor memory,wherein the stored value is a contact resistance value of a connectorthat is disposed between the monitor and the resistor when the monitorand the oximeter sensor are communicatively coupled, the contactresistance value being readable by the monitor to be used in acorrection mechanism of the monitor to correct the resistance valuebased on an algorithm that incorporates the resistance value and thecontact resistance value.